1. Technical Field
The present invention relates generally to sputtering, and more particularly to a reactive sputtering method and system for stabilizing the reactive sputtering process through sputter power control to balance the arrival and removal rates of reactive species at target surface for high-rate deposition of dielectric films.
2. Background Art
Dielectric and other non-conducting films are widely used in a broad range of applications such as semiconductor chips, magnetic and optical recording, flat panel displays, ink jet printer heads, solar cells, integrated optics, optical coatings, etc. Traditional deposition methods like RF diode and RF magnetron sputtering using a dielectric target have been well established and commonly used by the industries. However, their extremely low deposition rate prevents high-throughput production. Further, excessive substrate heating and poor thickness uniformity are normally associated with RF diode processes.
As an alternative, reactive sputtering which involves sputtering a metallic (elemental) target in Arxe2x80x94O2 or Arxe2x80x94N2 gas mixtures offers a much higher deposition rate, good film uniformity, as well as cooler substrate temperatures. In particular, the high deposition rate near the metal mode is very attractive for mass production of dielectric films. However, high-rate reactive sputtering near the metal mode is usually very unstable due to the poisoning of the target by the arriving oxygen or nitrogen. This can be described by the hysteresis curve that characterizes the deposition rate versus the flow rate of oxygen or nitrogen gas. To form dielectric films at high rate, reactive sputtering has to operate near or on the sharp falling edge of the curve. Any deviation from the equilibrium between the arrival and removal rates of oxygen at the target surface tends to escalate rapidly and result in a drastic drift towards the unwanted metal mode or the low-rate oxide mode. To prevent the drifting and hence stabilize the process, instant and effective response to any rate imbalance is mandatory.
Partial pressure control is the primary method used to maintain the stability of the reactive sputtering process. Any changes in oxygen or nitrogen partial pressures would impact the poisoning rate of the target and the combination rate with the metallic atoms to form oxide or nitride. This in turn affects many parameters like sputter voltage, discharge, current, intensity of plasma emission and optical emission from Al203 formation. Therefore, these parameters are often used as a monitor to control the feedback loop of partial pressure adjustment.
Due to the xe2x80x9cinertiaxe2x80x9d associated with pressure changes, partial pressure control appears not fast enough to correct the instability buildup during the deposition process. Too radical adjustment of partial pressure tends to overshoot and cause instability in the opposite direction. Moreover, the precision of partial pressure control may not be sufficient enough to match the subtle deviation from the balanced operating state. Other approaches include low-frequency modulation of discharge current and even pulsed DC power, but they are unable to follow the instant and random fluctuations near the operating point.
It may be appreciated, therefore, that further advancements and improvements in reactive sputter deposition are needed, particularly for providing effective an reliable stabilization of high-rate reactive sputtering for continuous deposition of dielectric films, and for providing instantaneous and precise response to any fluctuations that might cause instability of the sputter process.
The present invention provides such advancements and overcomes the above mentioned problems and other limitations, by providing a reactive sputtering processing system and method that controls power supplied to the cathode to stabilize the cathode voltage. For a given nominal cathode power level, target material, and source gases, the power supplied to the cathode (target) is controlled to stabilize the cathode (target) voltage at a specified value or within a specified range corresponding to a partial pressure or relative flow rate value or range of the reactive gas. Such an operating point or range, characterized by a specified voltage value or range and corresponding reactive gas relative-flow/partial-pressure value or range, may be determined empirically based on measuring the cathode voltage as a function of reactive gas relative-flow/partial-pressure for the given nominal power. This relationship is typically a hysteresis curve, and preferably the operation point is selected at or near the hysteresis transition edge to provide high rate deposition of high quality films, including insulating or dielectric films using a metallic target.
In accordance with an aspect of the present invention, a nominal flow rate or partial pressure of a reactive gas is established in a reactive sputter deposition system. The cathode voltage is stabilized at a nominal voltage by adjusting the power supplied to the cathode, wherein the nominal voltage and nominal flow rate or partial pressure of the reactive gas are determined according to an equilibrium state or steady state condition among the cathode voltage, the nominal flow rate or partial pressure of the reactive gas, and the power supplied to the cathode.
In accordance with a further aspect of the invention, the cathode voltage is stabilized at a nominal voltage by adjusting the power supplied to the cathode to balance the arrival and removal rate of said reactive species at the target.